Traditional signal transport systems include a sender and a receiver connected by a communications channel. The receiver includes a quantizer that converts the signal into digital form. The receiver needs to present the quantizer with adequate signal-to-noise ratio in order to maintain the desired level of bit errors.
In the case of a single-ended, or unbalanced, channel, the signal must be strong enough to overcome not only noise, but also the threshold uncertainty inherent in single-ended quantizers. Passive and active filter networks have been employed to compensate for channel-induced distortion so as to improve the signal-to-noise ratio in the middle of each bit cell time. These filter networks have the effect of increasing the timing uncertainty of the edges of the bit cell, thus causing jitter that must be removed by means of additional analog circuits, termed "retiming circuits" such as a resonant tank circuit or a phase-locked loop.
The analog retiming circuits typically require that the signal be encoded in such a way as to ensure a minimum transition density to carry the timing information. Non-return to zero representation of random binary data, while the simplest to implement, can not guarantee a transition density. Encoding is also required to allow the quantizer to find the average signal voltage that is used as the decision threshold. The analog retime circuit, as well as the encoder and decoder logic, add considerable complexity to the transport system.
The complexity of encoding and retiming is extremely undesirable in the case of large equipment that contains hundreds or thousands of signals needing transport. At high signal speeds, however, the simpler techniques of synchronous transport suffer from the effects of signal loss, distortion, and timing skew.
From the foregoing, it may be appreciated that a need exists for a high speed signal transport technique that is suitable for the high density and physical dispersion of much modern equipment, in particular telecommunications cross-connect systems.